Conventionally, (concentrated) sulfuric acid, fuming sulfuric acid, chlorosulfonic acid, sulfuric anhydride, adduct of dioxane with SO.sub.3, adduct of amine with SO.sub.3, etc. are chiefly used as agents for sulfonating aromatic compounds by introducing a sulfonic acid group into the aromatic ring of the compound. However, the use of such agents for sulfonation involves various problems. For example, sulfuric acid or fuming sulfuric acid, when used, produces water as a by-product to result in a reduced conversion. To preclude the reduction of the conversion, an excess of the sulfonating agent needs to be used, consequently producing a large amount of waste acid. On the other hand, use of chlorosulfonic acid or sulfuric anhydride produces no waste acid, whereas these agents have higher sulfonating and oxidating abilities than sulfuric acid and fuming sulfuric acid and therefore produce a sulfone or like by-product or form a colored sulfonated product. The adduct of dioxane or amine with SO.sub.3 is not as high as chlorosulfonic acid and sulfuric anhydride in sulfonating ability, but use of the adduct produces a waste acid and encounters difficulty in collecting the adduct.
Aromatic amine compounds are sulfonated by the so-called baking process wherein an acidic sulfate of amine is prepared from the aromatic amine compound and a stoichiometric amount of sulfuric acid and heated in a solid state or in a solvent at a high temperature for dehydration to obtain an aminesulfonic acid. The baking process also has the following problems. The process (solid process) wherein the salt is heated in the solid state to a high temperature is not usable for substances which are liable to oxidation, requires a special reactor and causes marked corrosion to the reactor. On the other hand, the process (solvent process) wherein the salt is heated in the solvent is applicable even to substances which are prone to oxidation, does not require a special reactor but is not substantially amenable to quantity production by a scale-up because if the process is practiced on an enlarged scale, the azeotropic dehydration with use of the solvent, which takes place in a velocity controlling step of the reaction, necessitates a greatly prolonged reaction time and causes oxidation of the product.